Yeah, if you look on the yarn label, it always specifies recommended needle size. I usually just compare it with the recommended needle size in the pattern.
Sometimes the yarn label is not clear if it's US size or metric. If the label is written in several languages, and doesn't specify US or metric, it's probably metric. (When in doubt, look for a yarn that looks similar, and check its label, or ask your LYS staff.)
This method isn't perfect though. I recently purchased some yarn that had a very small recommended needle size for the yarn's weight, which baffled me. The LYS staff said the yarn was designed to be used for a special type of knitting that is typically worked in a tight gauge. But that's not very common; usually recommended needle sizes are more standard.
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